Contrasting patterns of species and functional composition of coral reef sponge assemblages

There is mounting interest in studying the relationship between functional (FD) and species diversity (SD) in order to understand ecosystem functioning and develop effective measures to protect communities. Several studies have found strong positive correlations between SD and FD and in this study I used sponges to investigate this relationship and compare it with that of functional and species composition (FC and SC respectively). This study was conducted at 2 coral reef ecosystems in SE Sulawesi, Indonesia, which experience different sedimentation levels. Sponges fulfil a number of key functional roles on coral reefs, and, as many can be associated with specific morphologies, the measurement of morphological diversity (MD) should provide an approximation of FD. Previous sponge research has shown a strong correlation between MD and SD, and I hypothesise that spatial variation in MD, FD, SD and assemblage composition (FC and SC) will be determined by the same environmental gradients. Univariate measures of SD, MD and FD showed strong correlations, but not for multivariate data. Although different morphological assemblages (MC) were found at the 2 sites and between depth intervals, which correlated with SC data, no significant correlation was reported with FC data. FC was strongly influenced by depth, but not site, demonstrating that factors attributable to depth (e.g. light and wave action reduction) were more important in controlling FC, which contrasts with SC and MC patterns that were determined by sedimentation levels (i.e. between site differences). Although sponges fulfilled several functional roles, most were performed by a large number of morphologies/species, indicating a high level of intra-phyletic functional redundancy. The data from this study asserts that the factors responsible for spatial variation in SC may not always reflect those responsible for FC patterns, which has widespread implications for marine conservation, environmental management and Marine Protected Area (MPA) ecology. Environmental managers need to be aware of the factors controlling both SC and FC to ensure the protection of both biodiversity and ecosystem functioning.